Heating coils formed of helically wound resistance wire are widely employed in high temperature electrical furnaces. Typically such coils are supported by ceramic cores such as grooved plates or cylinders in which the heater is supported and confined throughout its entire length by the ceramic structure. The core usually has a plurality of longitudinal grooves formed in a surface thereof and surrounding the coil around a major portion of its periphery. Typically the groove shields approximately 270.degree. of the coil circumference such that the coil surface is exposed for radiation along a circumferential extent of 90.degree. or less. Alternately the groove is filled with a refractory sealing material such that the coil is fully embedded in a ceramic core. Another known coil heater employs a packed ceramic powder surrounding the heater coil and sheathed by a metal tube.
The weight of the ceramic support structure constitutes a major percentage of the overall heater assembly mass by reason of the amount of ceramic necessary for support of the heating coil and the inherent density of the ceramic material. Such ceramic support structures have relatively low thermal insulation properties and as a result of the relatively massive amount of ceramic material present, a coil heater of conventional construction exhibits a high thermal inertia which limits the rapidity with which a change of temperature can be accomplished. The response of such conventional heaters to temperature control is thereby limited by the relatively slow thermal response of the heater structure. The high thermal inertia also affects the overall efficiency of conventional heaters since the heat must saturate the surrounding ceramic material before direct radiation to the product can significantly occur. The ceramic core even in those conventional heaters having an open groove effectively shades all or a major portion of the direct radiation emitted by the coil thus providing a low emissivity, which in turn promotes a substantial differential in temperature between the product and the heating coil, causing inefficiency and shorter heater life. In those coil heaters employing a packed ceramic powder, air or gases often entrapped within the powder add to the insulative properties of the surrounding material with further unwanted shielding of the heater coil.